Silicon carbide (SiC)-based ceramic matrix composite (CMC) materials have been proposed as materials for certain components of gas turbine engines, such as the turbine blades, vanes, nozzles, and buckets. Various methods are known for fabricating SiC-based CMC components, including Silicomp, melt infiltration (MI), chemical vapor infiltration (CVI), polymer inflation pyrolysis (PIP), and oxide/oxide processes. Though these fabrication techniques significantly differ from each other, each involves the use of hand lay-up and tooling or dies to produce a near-net-shape part through a process that includes the application of heat at various processing stages.
As with turbine blades and vanes formed from more conventional superalloy materials, CMC blades and vanes are primarily equipped with cavities and cooling passages to reduce weight, reduce centrifugal load, and reduce operating temperatures of the components. These features are typically formed in CMC components using a combination of removable and expendable tooling.
Forming CMC component with a cavity includes a number of steps, including using pre-forms. First, a plurality of ceramic plies, some of which can include reinforcing material or are pre-impregnated with matrix, are laid up on a mandrel or mold in a pre-determined pattern to provide desired final or near-net-shape and desired mechanical properties of component. The mandrel is generally selected from various polymers, or other meltable materials. The laid-up plies may be pre-impregnated (pre-preg) with matrix material, such as SiC or impregnated with matrix after lay-up of the plies. Prior to rigidization of the CMC pre-form, the mandrel is removed through a burn-out cycle. In the burn-out cycle, the mandrel forming materials, such as, various polymers, or other meltable materials are melted out.
After the burn-out cycle, the CMC pre-form component is very fragile due to burn-off of the volatile substances of the composite. In certain cases, one end of the CMC pre-form requires capping or closing before use in gas turbines in order to close off the hollow region for use in the turbine. In known processes, to close the open end area of the CMC pre-form a cap or plug is inserted after the burnout cycle when the CMC pre-forms are in their most fragile state. The plug can be formed from of a CMC laminate part having a number of plies, and shaped so as to fill the open end of the CMC pre-form. Use of a separate forming, cutting and layup process is time and labor intensive simply to create a closed structure. Challenges also arise with placing the pre-rigidized CMC laminate having a number of plies into the open end. For example, because both the CMC laminate and pre-form are fragile prior to densification, these components can be easily damaged during assembly.
Therefore, a method of forming pre-form ceramic matrix composite cavity, a pre-form ceramic matrix composite cavity, and a method of forming ceramic matrix composite components that do not suffer from the above drawbacks is desirable in the art.